This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.

2024-10-26

1301. Value initialization of union

[Voted into the WP at the February, 2012 meeting; moved to DR at the October, 2012 meeting.]

According to 9.4 [dcl.init] paragraph 7,

To value-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type (Clause 11 [class]) with a user-provided constructor (11.4.5 [class.ctor]), then the default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);

• if T is a (possibly cv-qualified) non-union class type without a user-provided constructor, then the object is zero-initialized and, if T's implicitly-declared default constructor is non-trivial, that constructor is called.

• if T is an array type, then each element is value-initialized;

• otherwise, the object is zero-initialized.

This suggests that for

  struct A { A() = delete; };
  union B { A a };
  int main()
  {
    B();
  }

a B temporary is created and zero-initialized, even though its default constructor is deleted. We should strike "non-union" and also the "if...non-trivial" condition, since we can have a trivial deleted constructor.

Proposed resolution (August, 2011):

1. Change 9.4 [dcl.init] paragraph 7 as follows:

To value-initialize an object of type T means:

• if T is a (possibly cv-qualified) class type (Clause 11 [class]) with either no default constructor (11.4.5 [class.ctor]) or a default constructor that is user-provided or deleted constructor (11.4.5 [class.ctor]), then the object is default-initialized default constructor for T is called (and the initialization is ill-formed if T has no accessible default constructor);

• if T is a (possibly cv-qualified) non-union class type without a user-provided or deleted default constructor, then the object is zero-initialized and, if T's implicitly-declared default constructor is T has a non-trivial default constructor, that constructor is called. default-initialized;

• ...

2. Change 9.4.5 [dcl.init.list] paragraph 3 as follows:

List-initialization of an object or reference of type T is defined as follows:

• If the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.

• Otherwise, if If T is an aggregate, aggregate initialization is performed (9.4.2 [dcl.init.aggr]). [Example:...

• Otherwise, if the initializer list has no elements and T is a class type with a default constructor, the object is value-initialized.

• ...

This resolution also resolves issues 1324 and 1368.